The duplication of switching networks and other assemblies intervening in the establishment of connections between stations of telephone and other telecommunication systems enables the immediate substitution of a standby assembly for a normally used main assembly if the latter develops a malfunction. In order to avoid a switchover to a faulty standby assembly, it is necessary that both the main and the standby assembly be monitored continuously or periodically to determine their operating efficiency, for example. The testing of such equipment used in a time-sharing communication system may be carried out in special time slots not utilized for the transmission of information between subscribers, e.g. as described in U.S. Pat. No. 3,641,275. Thus, a processor may be programmed to send, at certain times, instructions to various ancillary units such as line testers monitoring the operation of the supervised assembly, these units in turn sending back data to the processor which on the basis thereof determines whether the assembly functions satisfactorily or needs to be replaced. For a correct determination of the utility of a switchover to a standby assembly it is, of course, important to ascertain whether the testing processor is itself free from malfunctions. If, for this purpose, duplicate testing processors are respectively assigned to the two interchangeable assemblies, checking these two processors with the aid of a third processor would greatly add to the complexity of the system.